A team of scientists has discovered that the iron-rich 'liquid fertiliser' created by whale faeces could return stability to the world's oceans. However, as Verica Jokic writes, whaling has severely depleted several species, and it could take centuries to return to a balanced ecosystem.

For about a decade scientists from around the world have been studying the crucial role large whales play in the health of marine ecosystems.

They've discovered whale faeces and rotting carcasses can return stability to the world's oceans by keeping plants and marine life fed.

The whales eat the krill and covert it into liquid iron and that comes out the back as liquid fertiliser.

Professor Stephen Nicol, University of Tasmania

As the plants absorb the liquid faeces and grow, they absorb more carbon, turning the oceans into large carbon sinks.

Professor Stephen Nicol from the University of Tasmania is one of 10 scientists involved in a new report conducted by the University of Vermont in the USA.

He says the whale poo theory had been around for a long time, but people didn't know what to do about it.

'A group of us in Hobart looked at how much iron there was in whale poo. Iron is important because there's very little of it in the Southern Ocean,' he says.

Professor Nicol says whales ingest krill that have iron concentrations 10 million times higher than the iron found in sea water.

'The whales eat the krill and convert it into liquid iron and that comes out the back as liquid fertiliser,' he says.

The study found whale movements through the water can disperse food to areas the food would otherwise not reach, and their rotting carcases provide nutrients for creatures that live on sea floors.

Large whale numbers have plummeted in the past century. Extensive hunting saw the number of blue whales reduced to one per cent of their original population. Blue fin whales were also targeted.

While blue whales can reproduce quickly (they can calve every two years), Professor Nicol says it will take decades or even centuries for their numbers to return to pre-whaling numbers.

'We now have an unbalanced ecosystem because the big and top level predators have been taken out, and the system is nothing like it used to be 100 years ago,' he says.

Professor Nicol says a stable ecosystem is one where all the various levels operate in the way they evolved.

He says that's a difficult argument that's difficult to sell, though, because it's contradicts the belief that ridding the ocean of top order fish will help fish numbers grow.